The reduction of CO2 emission is one of the greatest concerns in combating the catastrophic xe2x80x9cglobal warmingxe2x80x9d trend. As a result, the world puts much emphasis on the exploitation of xe2x80x9cclean energyxe2x80x9d with less or non-CO2 emission for both industrial and domestic uses. Natural gas (hereafter abbreviated as xe2x80x9cNGxe2x80x9d), as compared with coal and petroleum, is considered the most economic xe2x80x9ccleanxe2x80x9d fuel that could be used on a large, industrial scale at present and in the near future. In addition, the discovery of huge amount of ocean-bed gas-hydrate deposits substantially increases the recoverable NG resources. It is expected that, in the long run, the global NG consumption may eventually exceeds all other fossil fuels.
Dehydration is required for the exploitation, transportation, and processing of NG. The state-of-the-art technologies for NG dehydration include glycol absorption and molecular sieve adsorption. The glycol dehydrator is less expensive and widely used for moderate dewpoint reduction. The harmful emission of the BTEX (i.e., benzene, toluene, ethyl benzene, and xylene) from glycol dehydrators is the major environmental concern about this technology. The molecular sieve dehydrator is more expensive. Its application is limited to where higher dewpoint reduction is required. In addition, the larger size and weight of the molecular sieve dehydrator hinder its application to the off-shore platforms. An environmental-benign, compact, and economical dehydration technology, therefore, is desired to better serve the ever-booming NG industry.
Refrigeration dehydration technology, as a potential alternative to the above-mentioned dehydration technologies, has already been widely used for air dehumidification and compressed air dehydration. For NG dehydration, the major drawback is the clogging caused by solid ice/gas-hydrate depositions in the refrigeration dehydrator. More recently, technically breakthrough was proposed in both U.S. Pat. No. 5,664,426 xe2x80x9cRegenerative Gas Dehydratorxe2x80x9d (1997) and U.S. Pat. No. 6,158,242 xe2x80x9cGas Dehydration Methodology and Apparatusxe2x80x9d (2000). Following a successful field test of a refrigeration NG dehydrator prototype at a gas well in Texas (2000), active commercialization efforts on refrigeration NG dehydrator have been pursued both in the United States and in the PRC.
The current refrigeration dehydrator for NG operates on an alterative freezing-thawing cycle to eliminate the clogging of the solid ice/gas hydrate deposits in the flow channels and pipelines. Several identical moisture removal units have to be installed to allow such alternative operations. As a consequence, the size and cost of current refrigeration NG dehydrator could not be reduced to meet the requirements of a diversified market, in particular the off-shore and remote NG sites where a more compact equipment is required. A further breakthrough, therefore, in the refrigeration dehydration technology is desired.
Accordingly, it is an objective of the present invention to provide a non-frost deep-freezing refrigeration dehydrator wherein no solid ice/gas-hydrate depositions appear even at very low dewpoint. Alternative freezing-thawing operations are no longer required. Continuous operations are feasible with a single dehydration unit.
Another objective of the present invention is to provide a compact and light-weighted NG dehydrator for the applications to off-shore and remote NG sites.
Still another objective of the present invention is to provide an energy-saving refrigeration dehydrator that utilizes the expansion of the high-pressure NG to provide the required refrigeration.
A further objective of the present invention is to provide a high-efficiency free-piston expander-compressor to provide the required refrigeration.
With regard to the above and other objectives, the present invention provides a non-frost deep-freezing refrigeration dehydrator wherein no solid ice/gas-hydrate depositions appear even at very low dewpoint. Alternative freezing-thawing operations are no longer required. Continuous operations are feasible with a single dehydration unit.
The said apparatus consists of the following major components: a moisture-trap, i.e., a special heat exchanger comprising an upper pre-cooling section (hereafter abbreviated as xe2x80x9cpre-coolerxe2x80x9d) and a deep-cooling section (hereafter abbreviated as xe2x80x9cdeep-coolerxe2x80x9d); a gas-liquid separator; an inhibitor regenerator; and a refrigeration unit.
The principle of the operations of the non-frost deep-freezing refrigeration dehydrator follows. The application of the present invention to NG dehydration will be used as an example in the following descriptions, wherever appropriate.
The inlet moisture-laden NG enters of the moisture trap from the top of the primary side of the pre-cooler and flows downward all the way into the deep-cooler. The said inlet NG is first pre-cooled by the cold dehydrated NG reflux flowing upward through the secondary side of the pre-cooler, and then deep-cooled by the refrigerant (or brine) flowing through the secondary side of the deep-cooler. As the temperature of the inlet NG drops along its flow path, the moisture condenses on the surface of the flow channels, which is covered with a down-flowing liquid film of a gas-hydrate inhibitor (hereafter abbreviated as xe2x80x9cinhibitorxe2x80x9d) solution. The concentration of the inhibitor in the solution should be sufficiently high so that no solid deposit would appear in the liquid film all the way down to the NG outlet of the moisture trap. The dehydrated NG with desired dewpoint eventually exits from the bottom of the moisture trap.
The deep-cooled NG then enters a gas-liquid separator to clean up the entrained liquid droplets, if any. The fully dehydrated cold NG is recycled as a reflux coolant to the secondary side of the pre-heater.
The used inhibitor solution, diluted with the condensates, is sent to an inhibitor regenerator to be recovered as an enriched inhibitor solution. The latter is recycled. The produced wastewater is discharged.
The refrigeration unit provides the required refrigeration for the deep-cooler. In general, a separate industrial refrigerator could be used for this purpose. When the pressure of the inlet NG is sufficient high, the required refrigeration could be provided with expanding the dehydrated cold NG, preferably in a NG expander-compressor to recover a portion of the expansion energy. In such a xe2x80x9cself-refrigerationxe2x80x9d unit, no external energy is required for refrigeration.
In case that the pressure difference between the inlet NG and the NG transportation pipeline is small, a high-efficiency free-piston NG expander-compressor is proposed in the present invention to provide the self-refrigeration.